High discharge capacity lithium battery

ABSTRACT

Electrochemical battery cells, and more particularly, to cells comprising a lithium negative electrode and an iron disulfide positive electrode. Before use in the cell, the iron disulfide has an inherent pH, or a mixture of iron disulfide and an pH raising additive compound have a calculated pH, of at least a predetermined minimum pH value. In a preferred embodiment, the pH raising additive compound comprises a Group IIA element of the Periodic Table of the Elements, or an acid scavenger or pH control agent such as an organic amine, cycloaliphatic epoxy, amino alcohol or overbased calcium sulfonate. In one embodiment, the iron disulfide particles utilized in the cell have a specific reduced average particle size range. Methods for preparing cathodes and electrochemical battery cells comprising such cathodes including (a) iron disulfide or (b) a mixture of iron disulfide and the pH raising additive compound, having (a) an inherent pH or (b) a calculated pH greater than or equal to a predetermined minimum value are disclosed.

CROSS REFERENCE

This application is a continuation-in-part application of U.S.application Ser. No. 11/020,339, filed Dec. 22, 2004, now abandonedwhich is a continuation-in-part application of U.S. application Ser. No.10/719,425, filed Nov. 21, 2003, now abandoned both entitled “HIGHDISCHARGE CAPACITY LITHIUM BATTERY”, both herein fully incorporated byreference.

FIELD OF THE INVENTION

The present invention relates to electrochemical battery cells, moreparticularly to cells comprising a lithium negative electrode and aniron disulfide positive electrode, and methods for preparing cathodesand electrochemical battery cells are disclosed.

BACKGROUND OF THE INVENTION

Many electrical devices use electrochemical battery cells as powersources. The size and shape of the battery is often limited by thebattery compartment of the device. Manufacturers continually try toincrease the capabilities and features of electrical devices therebyincreasing demands on the batteries used therein. As the shape and sizeof the battery is often fixed, battery manufacturers must modify cellcharacteristics to provide increased performance.

Solutions to provide increased performance have included minimizingvolume taken up in the cell by the housing, including the seal and vent,as well as reducing the thickness of the separator between the negativeelectrode (anode) and positive electrode (cathode). Such solutionsattempt to maximize the internal volume available for active materials.

It is also desirable to utilize natural pyrite or ion disulfide (FeS₂)ore in the positive electrode of an electrochemical cell as an activematerial as it provides desirable performance characteristics. However,natural iron disulfide ore can contain any of a number of impurities,whether present in the natural product or added intentionally orunintentionally by a mining company or supplier. The type of impuritiescan vary by source, i.e. samples from different mining regions, or evenby lot from the same location. While sampling and lot selection can beutilized to limit impurity types or levels, or a combination thereof,impurities cannot be eliminated entirely. In the past, lithium/irondisulfide batteries were prepared having an iron disulfide averageparticle size greater than 20 μm, and a median inherent pH value priorto use in an electrode of about 5.0 with pH values ranging from 3.75 to6.86. Also, lithium/iron disulfide batteries were prepared having aniron disulfide average particle size from 1 to 19 μm, and a medianinherent pH value of about 4.75 with pH values ranging from 4.03 to5.56. Currently, the use of synthetic iron disulfide is costprohibitive.

The pyrite or iron disulfide particles utilized in electrochemical cellcathodes are typically derived from natural ore which is crushed, heattreated, and dry milled to an average particle size of 20 to 30 microns.The fineness of the grind is limited by the reactivity of the particleswith air and moisture. As the particle size is reduced, the surface areathereof is increased and is weathered. Weathering is an oxidationprocess in which the iron disulfide reacts with moisture and air to formiron sulfates. The weathering process results in an increase in acidityand a reduction in electrochemical activity. Small pyrite particles cangenerate sufficient heat during oxidation to cause hazardous fireswithin the processing operation. Prior art iron disulfide particlesutilized can have particles sizes which approach the final cathodecoating thickness of about 80 microns due to the inconsistencies of thedry milling process.

The dry milling process of iron disulfide is typically performed by amining company or an intermediate wherein large quantities of materialare produced. The processed iron disulfide is shipped and generallystored for extended periods of time before it can be used by the batteryindustry. Thus, during the storage period, the above-noted oxidation andweathering occur and the material degrades. Moreover, the large irondisulfide particles sizes can impact processes such as calendering,causing substrate distortion, coating to substrate bond disruption, aswell as failures from separator damage.

It has been found desirable to improve electrochemical battery cellperformance by utilizing smaller average particle size iron disulfide.The average particle size of the iron disulfide is typically reduced viaa milling process such as media or jet milling. As a result, the irondisulfide surface area is increased and additional impurity inclusionsare exposed and can be released into the electrolyte.

Batteries having iron disulfide positive electrodes and preferablylithium negative electrodes occasionally exhibit defects which result ininternal shorting. The defects are believed to be caused by impuritiessuch as metals, for example zinc, which are present in raw materialsources such as the iron disulfide. Some of the impurities are solublein the non-aqueous electrolyte and deposit on the negative electrode asdendrites. The dendrites can grow large enough to form a conductivebridge across the separator and cause an internal short in theelectrochemical cell.

For the foregoing reasons, there is a need for electrochemical cells inwhich internal short circuits or dendrite growth or formation can bereduced or substantially prevented. The prevention of dendrite growth orinternal short circuits should not come at the expense of cellperformance.

SUMMARY OF THE INVENTION

In view of the above problems and considerations, it is an object of thepresent invention to provide an electrochemical battery cell that hashigh capacity, performs well under expected temperature and operatingconditions, has a long storage life at a plurality of temperatures, andis not prone to failure as a result of internal short circuits.

It is also an object of the present invention to provide anelectrochemical battery cell that exhibits desirable discharge capacityand efficiency on both low and high power discharge.

Yet another object of the invention is to provide a Li/FeS₂ cell withincreased cathode interfacial capacity and having both improved energydensity and good resistance to internal short circuits.

It is an additional object of the present invention to provide anelectrochemical cell having a positive electrode comprising relativelysmall average particle size FeS₂ particles. A further object is toprovide an electrochemical cell having increased low and high rateproduct performance. Yet another object is to provide an electrochemicalcell which maintains a high voltage output for an extended period oftime. Still a further object of the invention is to provide methods forproducing electrochemical cells and especially a positive electrodetherefore with the method including the steps of forming a slurrycomprising FeS₂ particles and a wetting agent; utilizing a mill,particularly a media mill, to reduce the average particle size of theFeS₂ particles, and subsequently forming the positive electrodeutilizing the slurry. Another object of the present invention is toprovide electrochemical cells having a positive electrode comprisingiron disulfide particles which have been milled to a desired averageparticle size range utilizing a process such as jet milling, in whichsubstantially no heat is generated and a narrow particle sizedistribution is obtained.

Another object of the present invention is to provide an electrochemicalbattery cell having a positive electrode comprising (a) FeS₂ or (b) FeS₂and an optional pH raising additive compound, which prior to use in theelectrode has a pH of at least a predetermined minimum value. In oneembodiment, the FeS₂ has an average particle size from 1 to 19 μm. Inone aspect of the invention, the pH raising additive compound isprovided which comprises a Group IIA element of the Periodic Table ofthe Elements or an acid scavenger or pH control agent such as an organicamine, cycloaliphatic epoxy, amino alcohol or overbased calciumsulfonate.

An additional object of the present invention is to provide methods forpreparing a positive electrode for an electrochemical battery cell aswell as cells comprising the positive electrode, including the steps ofdetermining the pH of an FeS₂ sample, optionally adding a pH raisingadditive compound to increase the pH to at least a predetermined minimumvalue, and applying the FeS₂, and optionally the pH raising additivecompound to a cathode substrate. In one embodiment, the method formaking an electrochemical battery cell includes the steps of forming acell comprising the cathode comprising (a) FeS₂ having an inherent pH,or (b) a mixture comprising FeS₂ and the pH raising additive compoundhaving a calculated pH, greater than or equal to a predetermined minimumvalue.

The above objects are met and the above disadvantages of the prior artare overcome by the present invention.

One aspect of the invention is directed to an electrochemical batterycell, comprising a housing; a negative electrode comprising lithium; apositive electrode comprising either (a) iron disulfide having aninherent pH from 6.0 to 14.0, or (b) a mixture of iron disulfide and apH raising additive compound, wherein the pH raising additive compoundis present in an effective amount to provide the mixture with acalculated pH of 5.0 to 14.0; and a non-aqueous electrolyte mixturecomprising at least one salt dissolved in a solvent disposed within thehousing.

Yet another aspect of the invention is directed to an electrochemicalbattery cell, comprising a housing; a negative electrode comprisinglithium; a positive electrode comprising either (a) iron disulfidehaving an inherent pH from 5.0 to 14.0 prior to use in the cell, or (b)a mixture of iron disulfide and a pH raising additive compound, whereinthe pH raising additive compound is present in an effective amount toprovide the mixture with a calculated pH of 5.0 to 14.0, wherein theiron disulfide has an average particle size from 1 to 19 μm; and anon-aqueous electrolyte mixture comprising at least one salt dissolvedin a solvent disposed within the housing.

Yet another aspect of the invention is directed to an electrochemicalbattery cell, comprising a housing; a negative electrode comprisinglithium; a positive electrode comprising iron disulfide and a pH raisingadditive compound comprising a Group IIA element, an organic amine, acycloaliphatic epoxide, an amino alcohol, an overbased metal sulfonate,or a combination thereof, wherein the positive electrode is free oflithium carbonate if calcium hydroxide is present as the pH raisingadditive compound; a non-aqueous electrolyte mixture comprising at leastone salt dissolved in a solvent disposed within the housing; and aseparator disposed between the negative electrode and the positiveelectrode.

Yet another aspect of the invention is directed to a process for makinga cathode, comprising the steps of obtaining iron disulfide; determininga pH of the iron disulfide; and (a) if the iron disulfide pH is from 6.0to 14.0 and an average iron disulfide particle size is 20 to 30 μm,forming a cathode using the iron disulfide; and (b) if the irondisulfide pH is from 5.0 to 14.0 and an average iron disulfide particlesize is 1 to 19 μm, forming a cathode using the iron disulfide. Anelectrochemical cell can be made by combining a cathode made accordingto this process with lithium and a separator situated between the anodeand the cathode, adding a non-aqueous electrolyte, and sealing theanode, cathode, separator and electrolyte in a cell housing.

Yet another aspect of the invention is directed to a process for makinga cathode, comprising the steps of obtaining iron disulfide; adding a pHraising additive compound to the iron disulfide in an effective amountso that a calculated pH of the iron disulfide and pH raising additivecompound is 4.0 to 14.0; and applying the iron disulfide and the pHraising additive compound to a cathode substrate. An electrochemicalcell can be made by combining a cathode made according to this processwith an anode comprising lithium and a separator situated between theanode and cathode, adding a non-aqueous electrolyte, and sealing theanode, cathode, separator and electrolyte in a cell housing.

These and other features, advantages and objects of the presentinvention will be further understood and appreciated by those skilled inthe art by reference to the following specification, claims and appendeddrawings.

Unless otherwise specified, as used herein the terms listed below aredefined as follows:

-   -   active material—one or more chemical compounds that are part of        the discharge reaction of a cell and contribute to the cell        discharge capacity, including impurities and small amounts of        other moieties present;    -   active material mixture—a mixture of solid electrode materials,        excluding current collectors and electrode leads, that contains        the electrode active material;    -   average particle size—the mean diameter of the volume        distribution of a sample of a composition (MV); can be measured        using a Microtrac Honeywell Particle Size Analyzer Model X-100        equipped with a Large Volume Recirculator (LVR) (4 L Volume)        Model 9320. The measuring method utilizes sonification to break        up agglomerates and prevent re-agglomeration. A sample of about        2.0 grams is weighed and placed into a 50 ml beaker. 20 ml of        deionized water and 2 drops of surfactant (1% Aerosol OT        solution prepared from 10 ml 10% Aerosol OT available from        Fisher Scientific in 100 mls deionized water with the solution        being well mixed). The beaker sample solution is stirred,        preferably with a stirring rod. The Large Volume Recirculator is        filled to level with deionized water and the sample is        transferred from the beaker to the Recirculator bowl. A wash        bottle is used to rinse out any remaining sample particles into        the Recirculator bowl. The sample is allowed to recirculate for        one minute before measurements are started. The following        parameters are input for FeS₂ particles: Transparent        Particles—No (absorbing); Spherical Particles—No; Fluid        Refractive Index—1.33; Run Time—60 seconds;    -   calculated pH of a mixture of FeS₂ and a pH raising additive        compound—a pH value calculated based on the inherent pH of the        FeS₂ and the relative amounts of the FeS₂ and the pH raising        additive compound;    -   capacity, discharge—the actual capacity delivered by a cell        during discharge, generally expressed in amp-hour (Ah) or        milliamp-hours (mAh);    -   capacity, input—the theoretical capacity of an electrode, equal        to the weight of each active material in the electrode times the        theoretical specific capacity of that active material, where the        theoretical specific capacity of each active material is        determined according to the following calculation:        [(96,487 ampere-seconds/mole)/(number of grams/mole of active        material)]×(number of electrons/mole of active material)/(3600        seconds/hour)×(1000 milliampere hours/ampere-hour)    -   (e.g., Li=3862.0 mAh/g, S=1672.0 mAh/g, FeS₂=893.6 mAh/g,        CoS₂−871.3 mAh/g, CF_(x)=864.3 mAh/g, CuO=673.8 mAh/g, C₂F=623.0        mAh/g, FeS=609.8 mAh/g, CuS=560.7 mAh/g, Bi₂O₃=345.1 mAh/g,        MnO₂=308.3 mAh/g, Pb₂Bi₂O₅=293.8 mAh/g and FeCuS₂−292.1 mAh/g);    -   capacity, cell interfacial—the smaller of the negative and        positive electrode capacity;    -   capacity, electrode interfacial—the total contribution of an        electrode to the cell theoretical discharge capacity, based on        the overall cell discharge reaction mechanism(s) and the total        amount of active material contained within the that portion of        the active material mixture adjacent to active material in the        opposite electrode, assuming complete reaction of all of the        active material, generally expressed in Ah or mAh (where only        one of the two major surfaces of an electrode strip is adjacent        active material in the opposite electrode, only the active        material on that side of the electrode—either the material on        that side of a solid current collector sheet or that material in        half the thickness of an electrode without a solid current        collector sheet—is included in the determination of interfacial        capacity);    -   electrode assembly—the combination of the negative electrode,        positive electrode, and separator, as well as any insulating        materials, overwraps, tapes, etc., that are incorporated        therewith, but excluding any separate electrical lead affixed to        the active material, active material mixture or current        collector;    -   electrode gap—the distance between adjacent negative and        positive electrodes;    -   electrode loading—active material mixture dry weight per unit of        electrode surface area, generally expressed in grams per square        centimeter (g/cm²);    -   electrode packing—active material dry weight per unit of        electrode surface area divided by the theoretical active        material mixture dry weight per unit of electrode surface area,        based on the real densities of the solid materials in the        mixture, generally expressed as a percentage;    -   pH of (s) FeS₂ or (b) a mixture of FeS₂ and a pH raising        additive compound—a pH of (a) FeS₂ particles or (b) a mixture of        FeS₂ particles and a pH raising additive compound, respectively,        suspended in water, which can be determined by the following        method: (1) place CO₂-free deionized water into a beaker; (2)        while stirring the water, pH is measured with a pH meter and        adjust as necessary to a pH of 6.9 to 7.1 with dilute NaOH        solution; (3) place 5.000 g sample of (a) FeS₂ or (b) a mixture        of FeS₂ and pH raising additive compound into a 100 ml beaker,        and add 50 ml of the pH-adjusted water; and (4) while stirring        the same and water (vigorously enough to maintain the majority        of the same in suspension without causing the water to        cavitate), measure the pH at 30-second intervals until it        stabilizes, recording the stable pH value as the pH of (a) the        FeS₂ or (b) the mixture of FeS₂ and pH raising additive        compound;    -   folded electrodes—electrode strips that are combined into an        assembly by folding, with the lengths of the strips either        parallel to or crossing one another;    -   inherent pH—pH of material received from a mining company or        supplier independent of battery manufacturing, can fall with the        passage of time;    -   interfacial height, electrode assembly—the average height,        parallel to the longitudinal axis of the cell, of the        interfacial surface of the electrodes in the assembly;    -   interfacial volume, electrode assembly—the volume within the        cell housing defined by the cross-sectional area, perpendicular        to the longitudinal axis of the cell, at the inner surface of        the container side wall(s) and the electrode assembly        interfacial height;    -   nominal—a value, specified by the manufacturer, that is        representative of what can be expected for that characteristic        or property;    -   percent discharge—the percentage of the rated capacity removed        from a cell during discharge;    -   room temperature—between about 20° C. and about 25° C.;    -   spiral wound electrodes—electrode strips that are combined into        an assembly by winding along their lengths or widths, e.g.,        around a mandrel or central core; and    -   void volume, electrode assembly—the volume of the electrode        assembly voids per unit of interfacial height, determined by        subtracting the sum of the volumes of the non-porous electrode        assembly components and the solid portions of the porous        electrode assembly components contained within the interfacial        height from the electrode assembly interfacial volume        (microporous separators, insulating films, tapes, etc. are        assumed to be non-porous and non-compressible, and volume of a        porous electrode is determined using the real densities of the        components and the total actual volume), generally expressed in        cm³/cm.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be better understood and other features andadvantages will become apparent by reading the detailed description ofthe invention, taken together with the drawings, wherein:

FIG. 1 is an embodiment of the electrochemical battery cell of theinvention;

FIG. 2 is a graph showing Impact Test results for partially dischargedFR6 cells as a function of the volume of voids per unit height of theelectrode assembly within the interfacial height;

FIG. 3 a illustrates a SEM micrograph at 1,000 times magnification of aportion of a positive electrode containing prior art FeS₂ particles;

FIG. 3 b illustrates a SEM micrograph at 1,000 times magnification of aportion of a positive electrode containing FeS₂ particles producedutilizing the media milling process of the invention;

FIG. 4 is a plot of cathode efficiency on DSC application as a functionof separator thickness for sets of FR6 type cells constructed havingvarying separator thickness, average particle size of FeS₂, andelectrolyte composition;

FIG. 5 is a graph of anode voltage as a function of percent depth ofdischarge for a prior art FeS₂-containing electrochemical cell, a cellcontaining media milled FeS₂ particles and a cell containing jet milledFeS₂ particles; and

FIG. 6 is a graph of cell voltage as a function of percent depth ofdischarge for a prior art FeS₂-containing electrochemical cell, a cellcontaining media milled FeS₂ particles and a cell containing jet milledFeS₂ particles.

DESCRIPTION OF THE INVENTION

The battery cell of the invention has an anode comprising metalliclithium as the negative electrode active material. The anode and cathodeare both in the form of strips, which are joined together in anelectrode assembly to provide a high interfacial surface area relativeto the volumes of the electrodes containing active material. The higherthe interfacial surface area, the lower the current density and thebetter the cell's capability to deliver high power on discharge. Thecell also has a high ratio of cathode interfacial capacity to electrodeassembly interfacial volume—at least 710 mAh/cm². This means that thevolume of active materials in the electrode assembly is high, to providea high discharge capacity. The high volume of active materials can beachieved by controlling a number of variables, including: the ratio ofinterfacial input capacity to total input capacity, the volume of thecathode current collector, the concentration of active cathode materialin the cathode mixture and the volume of separator in the electrodeassembly.

The invention will be better understood with reference to FIG. 1, whichshows an embodiment of a cell according to the invention. Cell 10 is anFR6 type cylindrical Li/FeS₂ battery cell. Cell 10 has a housing thatincludes a can 12 with a closed bottom and an open top end that isclosed with a cell cover 14 and a gasket 16. The can 12 has a bead orreduced diameter step near the top end to support the gasket 16 andcover 14. The gasket 16 is compressed between the can 12 and the cover14 to seal an anode 18, a cathode 20 and electrolyte within the cell 10.The anode 18, cathode 20 and a separator 26 are spirally wound togetherinto an electrode assembly. The cathode 20 has a metal current collector22, which extends from the top end of the electrode assembly and isconnected to the inner surface of the cover 14 with a contact spring 24.The anode 18 is electrically connected to the inner surface of the can12 by a metal tab (not shown). An insulating cone 46 is located aroundthe peripheral portion of the top of the electrode assembly to preventthe cathode current collector 22 from making contact with the can 12,and contact between the bottom edge of the cathode 20 and the bottom ofthe can 12 is prevented by the inward-folded extension of the separator26 and an electrically insulating bottom disc 44 positioned in thebottom of the can 12. Cell 10 has a separate positive terminal cover 40,which is held in place by the inwardly crimped top edge of the can 12and the gasket 16. The can 12 serves as the negative contact terminal.Disposed between the peripheral flange of the terminal cover 40 and thecell cover 14 is a positive temperature coefficient (PTC) device 42 thatsubstantially limits the flow of current under abusive electricalconditions. Cell 10 also includes a pressure relief vent. The cell cover14 has an aperture comprising an inward projecting central vent well 28with a vent hole 30 in the bottom of the well 28. The aperture is sealedby a vent ball 32 and a thin-walled thermoplastic bushing 34, which iscompressed between the vertical wall of the vent well 28 and theperiphery of the vent ball 32. When the cell internal pressure exceeds apredetermined level, the vent ball 32, or both the ball 32 and bushing34, is forced out of the aperture to release pressurized gases from thecell 10.

The cell container is often a metal can with an integral closed bottom;though a metal tube that is initially open at both ends may also be usedinstead of a can. The can is generally steel, plated with nickel on atleast the outside to protect the outside of the can from corrosion. Thetype of plating can be varied to provide varying degrees of corrosionresistance or to provide the desired appearance. The type of steel willdepend in part on the manner in which the container is formed. For drawncans the steel can be a diffusion annealed, low carbon, aluminum killed,SAE 1006 or equivalent steel, with a grain size of ASTM 9 to 11 andequiaxed to slightly elongated grain shape. Other steels, such asstainless steels, can be used to meet special needs. For example, whenthe can is in electrical contact with the cathode, a stainless steel maybe used for improved resistance to corrosion by the cathode andelectrolyte.

The cell cover is typically metal. Nickel plated steel may be used, buta stainless steel is often desirable, especially when the cover is inelectrical contact with the cathode. The complexity of the cover shapewill also be a factor in material selection. The cell cover may have asimple shape, such as a thick, flat disk, or it may have a more complexshape, such as the cover shown in FIG. 1. When the cover has a complexshape like that in FIG. 1, a type 304 soft annealed stainless steel withASTM 8-9 grain size may be used, to provide the desired corrosionresistance and ease of metal forming. Formed covers may also be plated,with nickel for example.

The terminal cover should have good resistance to corrosion by water inthe ambient environment, good electrical conductivity and, when visibleon consumer batteries, an attractive appearance. Terminal covers areoften made from nickel plated cold rolled steel or steel that is nickelplated after the covers are formed. Where terminals are located overpressure relief vents, the terminal covers generally have one or moreholes to facilitate cell venting.

The gasket is made from any suitable thermoplastic material thatprovides the desired sealing properties. Material selection is based inpart on the electrolyte composition. Examples of suitable materialsinclude polypropylene, polyphenylene sulfide,tetrafluorideperfluoroalkyl vinylether copolymer, polybutyleneterephthalate and combinations thereof. Preferred gasket materialsinclude polypropylene (e.g., PRO-FAX® 6524 from Basell Polyolefins,Wilmington, Del., USA), polybutylene terephthalate (e.g., CELANEX® PBT,grade 1600A from Ticona-US, Summit, N.J., USA) and polyphenylene sulfide(e.g., TECHTRON® PPS from Boedeker Plastics, Inc., Shiner, Tex, USA).Small amounts of other polymers, reinforcing inorganic fillers and/ororganic compounds may also be added to the base resin of the gasket.

The gasket may be coated with a sealant to provide the best seal.Ethylene propylene diene terpolymer (EPDM) is a suitable sealantmaterial, but other suitable materials can be used.

The vent bushing is made from a thermoplastic material that is resistantto cold flow at high temperatures (e.g., 75° C.). The thermoplasticmaterial comprises a base resin such as ethylene-tetrafluoroethylene,polybutylene terephthlate, polyphenylene sulfide, polyphthalamide,ethylenechloro-trifluoroethylene, chlorotrifluoroethylene,perfluoro-alkoxyalkane, fluorinated perfluoroethylene polypropylene andpolyetherether ketone. Ethylene-tetrafluoroethylene copolymer (ETFE),polyphenylene sulfide (PPS), polybutylene terephthalate (PBT) andpolyphthalamide are preferred. The resin can be modified by adding athermal-stabilizing filler to provide a vent bushing with the desiredsealing and venting characteristics at high temperatures. The bushingcan be injection molded from the thermoplastic material. TEFZEL® HT2004(ETFE resin with 25 weight percent chopped glass filler) is a preferredthermoplastic material.

The vent ball can be made from any suitable material that is stable incontact with the cell contents and provides the desired cell sealing andventing characteristic. Glasses or metals, such as stainless steel, canbe used.

The anode comprises a strip of lithium metal, sometimes referred to aslithium foil. The composition of the lithium can vary, though forbattery grade lithium the purity is always high. The lithium can bealloyed with other metals, such as aluminum, to provide the desired cellelectrical performance. Battery grade lithium-aluminum foil containing0.5 weight percent aluminum is available from Chemetall Foote Corp.,Kings Mountain, N.C., USA.

The anode may have a current collector, within or on the surface of themetallic lithium. As in the cell in FIG. 1, a separate current collectormay not be needed, since lithium has a high electrical conductivity, buta current collector may be included, e.g., to maintain electricalcontinuity within the anode during discharge, as the lithium isconsumed. When the anode includes a current collector, it may be made ofcopper because of its conductivity, but other conductive metals can beused as long as they are stable inside the cell.

A thin metal strip often serves as an electrical lead, or tab,connecting the anode to one of the cell terminals (the can in the caseof the FR6 cell shown in FIG. 1). The metal strip is often made fromnickel or nickel plated steel and affixed directly to the lithium. Thismay be accomplished embedding an end of the lead within a portion of theanode or by simply pressing an end of the lead onto the surface of thelithium foil.

The cathode is in the form of a strip that comprises a current collectorand a mixture that includes one or more electrochemically activematerials, usually in particulate form. Iron disulfide (FeS₂) is apreferred active material. In a Li/FeS₂ cell the active materialcomprises greater than 50 weight percent FeS₂. The cathode can alsocontain one or more additional active materials, depending on thedesired cell electrical and discharge characteristics. The additionalactive cathode material may be any suitable active cathode material.Examples include Bi₂O₃, C₂F, CF_(x), (CF)_(n), CoS₂, CuO, CuS, FeS,FeCuS₂, MnO₂, Pb₂Bi₂O₅ and S. More preferably the active material for aLi/FeS₂ cell cathode comprises at least 95 weight percent FeS₂, yet morepreferably at least 99 weight percent FeS₂, and most preferably FeS₂ isthe sole active cathode material. Battery grade FeS₂ having a puritylevel of at least 95 weight percent is available from American Minerals,Inc., Camden, N.J., USA; Chemetall GmbH, Vienna, Austria; WashingtonMills, North Grafton, Mass.; and Kyanite Mining Corp., Dillwyn, Va.,USA.

In addition to the active material, the cathode mixture contains othermaterials. A binder is generally used to hold the particulate materialstogether and adhere the mixture to the current collector. One or moreconductive materials such as metal, graphite and carbon black powdersmay be added to provide improved electrical conductivity to the mixture.The amount of conductive material used can be dependent upon factorssuch as the electrical conductivity of the active material and binder,the thickness of the mixture on the current collector and the currentcollector design. Small amounts of various additives may also be used toenhance cathode manufacturing and cell performance. The following areexamples of active material mixture materials for Li/FeS₂ cell cathodes.Graphite: KS-6 and TIMREX® MX15 grades synthetic graphite from TimcalAmerica, Westlake, Ohio, USA. Carbon black: Grade C55 acetylene blackfrom Chevron Phillips Company LP, Houston, Tex., USA. Binder:ethylene/propylene copolymer (PEPP) made by Polymont Plastics Corp.(formerly Polysar, Inc.) and available from Harwick StandardDistribution Corp., Akron, Ohio, USA; non-ionic water solublepolyethylene oxide (PEO): POLYOX® from Dow Chemical Company, Midland,Mich., USA; and G1651 grade styrene-ethylene/butylenes-styrene (SEBS)block copolymer from Kraton Polymers, Houston, Tex. Additives: FLUO HT®micronized polytetrafluoroethylene (PTFE) manufactured by Micro PowdersInc., Tarrytown, N.Y., USA (commercially available from Dar-Tech Inc.,Cleveland, Ohio, USA) and AEROSIL® 200 grade fumed silica from DegussaCorporation Pigment Group, Ridgefield, N.J.

The current collector may be disposed within or imbedded into thecathode surface, or the cathode mixture may be coated onto one or bothsides of a thin metal strip. Aluminum is a commonly used material. Thecurrent collector may extend beyond the portion of the cathodecontaining the cathode mixture. This extending portion of the currentcollector can provide a convenient area for making contact with theelectrical lead connected to the positive terminal. It is desirable tokeep the volume of the extending portion of the current collector to aminimum to make as much of the internal volume of the cell available foractive materials and electrolyte.

A preferred method of making FeS₂ cathodes is to roll coat a slurry ofactive material mixture materials in a highly volatile organic solvent(e.g., trichloroethylene) onto both sides of a sheet of aluminum foil,dry the coating to remove the solvent, calender the coated foil tocompact the coating, slit the coated foil to the desired width and cutstrips of the slit cathode material to the desired length. It isdesirable to use cathode materials with small particle sizes to minimizethe risk of puncturing the separator. For example, FeS₂ is preferablysieved through a 230 mesh (63 μm) screen before use. Coating thicknessesof 100 μm and less are common.

In a further embodiment, a cathode or positive electrode is disclosedwhich provides beneficial properties to an electrochemical cellincorporating the same therein. The cathode comprises FeS₂ particleshaving a predetermined average particle size produced by a wet millingmethod such as a media mill, or a dry milling method using anon-mechanical milling device such as a jet mill. Electrochemical cellsprepared with the reduced average particle size FeS₂ particles exhibitincreased cell voltage at any given depth of discharge, irrespective ofcell size. The smaller FeS₂ particles also make possible thinnercoatings of cathode material on the current collector; for example,coatings as thin as about 10 μm can be used.

In one embodiment of the present invention, the cathode comprises smallparticle size FeS₂ particles, preferably natural, produced by a wetmilling method, preferably utilizing a media mill. A media mill has alsobeen referred to in the art as a ball mill, basket mill, bead mill, sandmill, rotary-tumbling mixer, or the like, which can use milling media ina wet milling process. The wet milling step is preferably performedin-line during cathode or positive electrode construction therebysubstantially eliminating weathering or oxidation, as well as hazardousdry dust pyrite fires. By utilizing the wet milling process of thepresent invention, the above noted sieving operation can be eliminated.

In the wet milling method, a cathode electrochemically active materialmixture is formed comprising the FeS₂ and a wetting agent. At this pointin the process, the FeS₂ has an average particle size greater than 20μm. Any of the above described active or inactive materials such as, butnot limited to, binders, conductive material, additives, etc. can alsobe utilized in the active material mixture, if desired. In oneembodiment, the cathode active material mixture components are combinedand optionally, but preferably, mixed in a suitable vessel. The cathodeactive material mixture is metered into the media mill wherein theaverage particle size of the FeS₂ particles is reduced during milling.The dwell time of the cathode active material mixture within the mediamill is sufficient to produce the desired FeS₂ average particle sizerange.

The wetting agent is any liquid or the like, preferably of a lowviscosity, which substantially prevents the FeS₂ or the other componentsof the slurry from combusting during the milling process. The preferredwetting agent is a solvent which is generally non-flammable atprocessing conditions used during the wet milling operation. Examples ofsuitable wetting agents include, but are not limited totrichloroethylene, N-methyl-2-pyrrolidone (NMP), butyl glycol acetate,mineral spirits, and water. The wetting agent is selected to at least becompatible with and preferably able to substantially dissolve the binderutilized in preparation of the cathode. The amount of wetting agent canvary, and can generally range from about 0.1 cc to about 5 cc, andpreferably is about 0.5 cc per gram of solid components of the cathodeactive material mixture.

The cathode active material slurry mixture is transferred to a millingdevice and milled at an appropriate flow rate and rotor rpm until thedesired FeS₂ average particle size is achieved. A media mill is utilizedin a preferred embodiment. Media mills typically comprise shaft mountedrotating disks and/or rotors as well as grinding media in order toreduce particle size of components of the composition to be milled.Grinding media can be substantially spherical, cylindrical or the like,with spheres being preferred, with mean diameters which range from about0.2 mm to about 30 mm, and desirably about 0.5 to about 10 mm, andpreferably from about 1.2 to about 1.7 mm. Cylinder height ranges fromabout 1 mm to about 20 mm with about 5 to about 15 mm preferred.Numerous types of media can be utilized and include, but are not limitedto, soda lime, zirconia-silica, alumina oxide, yittria stabilizedzirconia silica, chrome steel, zirconium silicate, cerium stabilizedzirconia, yittria stabilized zirconia, and tungsten carbide. Suitablegrinding media are available from suppliers such as Saint-Gobain ofWorcester, Mass. as Glass, ER120, Zirstar and Zirmil; Glenn Mill ofCliffton, N.J. as Alumina, Steel, and Carbide; and Jyoti CeramicIndustries of Satpur, Nashik, India as Zirconox and Zircosil. A suitablemedia mill is available from Morehouse-COWLES of Fullerton, Calif.

The cathode active material slurry mixture is transferred to the millingchamber of the media mill which contains grinding media and preferablyshaft mounted rotatable rotors. The media is accelerated at a relativelyhigh velocity through the slurry towards the milling chamber wallthereby impacting, shearing, and reducing the size of the slurry mixtureparticles. The milled slurry mixture is subsequently discharged from themedia mill for further processing into a cathode after a desired averageparticle size of FeS₂ particles has been achieved.

After processing utilizing the wet milling method of the invention, theFeS₂ particles have an average particle size of about 1 to about 19 μm,desirably from about 2 to about 17 or about 18 μm and preferably fromabout 5 or about 10 to about 15 μm. The FeS₂ particles also have anarrower particle size distribution due to the media milling processperformed thereon.

The wet milled active cathode material mixture is subsequently rollcoated on a sheet such as aluminum foil as described hereinabove, anddried to remove the wetting agent. The coated foil laminate can then becalendered to compact the coating and produce a smooth surface, and thecoated foil can be slit to a desired width and length for use in theassembly of an electrochemical cell, such as described herein.

In a further embodiment of the present invention, the cathode comprisesFeS₂ particles, preferably natural, of a predetermined average particlesize range obtained by a non-mechanical milling device, preferably a jetmill. The term “non-mechanical milling device” refers to an apparatuswhich does not utilize pressure or contact between two or more millsurfaces to reduce the particle size of a material such as by crushing,chipping, fracturing, or the like. Mechanical milling devices include,but are not limited to, roll mills, granulating mills, ball mills, mediamills, bead mills, and hammer mills. Non-mechanical milling devicestypically reduce average particle size of the FeS₂ particles withoututilizing moving milling parts, and instead reduce size utilizingcollisions between particles and/or particles and a single surface ofthe milling device.

A jet mill typically includes a central chamber into which a fluid suchas air, steam, or gas is introduced through nozzles or jets which createa near-sonic, sonic or supersonic grinding stream. No grinding media areutilized. Particles of the feed material comprising FeS₂ particles arefed or injected into the high speed grinding stream in the jet mill.Size reduction results due to the high velocity collisions betweenparticles of the iron disulfide or other particles themselves orcollision with a mill surface. Jet mills are designed to allowrecirculation of oversized particles, enhancing the incidence and effectof particle collisions. As the FeS₂ particles are reduced in size, theymigrate towards a discharge port from which they are collected for usein an active material mixture utilized to form a cathode. In a preferredembodiment, the jet milling of the FeS₂ is performed in a inertatmosphere utilizing a gas such as nitrogen, argon, or the like withnitrogen being most preferred, in order to prevent ignition orcombustion of the FeS₂ particles. Although heat may be generated by thefriction of the FeS₂ particles rubbing over mill surfaces and from thecollisions taking place in the mill, due at least to the Jewel-Thompsoneffect on air temperature when throttling, there is reportedly no nettemperature increase during milling. The product temperature issubstantially equal to the temperature of the fluid supplied to themill. Jet mills are available from the Jet Pulverizer Company ofMoorestown, N.J.; Sturtevant of Hanover, Mass.; as well as Fluid Energyof Telford, Pa.

After processing utilizing the non-mechanical or jet milling method ofthe invention, the FeS₂ particles have an average particle size of about1 to about 19 μm, desirably from about 1.5 to about 10 or about 15 μm,and preferably from about 2 to about 6 μm. The jet milled FeS₂ particleshave a particle size distribution wherein 80% of the total particles arebetween about 1.0 and about 15 μm, and preferably about 1.0 and about 10μm. Particle size distribution was determined utilizing the MicrotracHoneywell Particle Size Analyzer X-100 described herein above, whereinsonification is utilized during testing in order to prevent aggregationof particles.

The milling processes of the present invention utilized to reduce theaverage particle size of the FeS₂ particles within the ranges statedherein have been shown to offer several advantages which include forexample, improved low temperature battery performance, improved adhesionof the cathode active material mixture to the aluminum substrate, lessdamage to the polymer separator insulator film due to the small particlesizes of the active material mixture, improved cathode efficiency as aresult of more pyrite particles with increased surface area to acceptlithium ions upon cell discharge, improved cell operating voltage fromdecreased anode polarization which allows the cells to operate at lowercurrents on constant power device applications, and more efficient anduniform discharge at the opposing lithium anode as the currentdistribution can be more uniformly applied over it's interfacial surfacearea.

FR6 type electrochemical cells prepared utilizing wet milled FeS₂particles or jet milled FeS₂ particles are capable of providing adischarge capacity of at least 3,000 milliamp-hours (mAh) whendischarged at a rate of 200 mA continuously to one volt, as well as atleast 2,700 mAh or preferably at least 2,800 mAh when discharged at arate of 1 amp continuously to one volt at room temperature. Accordingly,the cells of the present invention provide excellent results for bothlow and high rate applications.

It has also been found that FR6 electrochemical cells utilizing jetmilled FeS₂ particles as disclosed in the present invention have adischarge time generally of at least 300 minutes, desirably at least 320minutes, preferably at least 325 minutes, and most preferably at least330 or 340 minutes to 1.05 volts according to a 1500/650 mW 2/28 s×10per hour DSC test. It has also been found that FR6 type electrochemicalcells comprising jet milled FeS₂ particles, having an average particlesize within the range specified in the invention, maintain a voltage≧1.2 for at least 180 minutes, desirably at least 240 minutes, andpreferably at least 270 minutes according to the 1500/650 mW 2/28 s×10per hour DSC test. The DSC procedure cycles the electrochemical cellutilizing two pulses, the first pulse at 1500 mW for 2 seconds followedby the second pulse at 650 mW for 28 seconds. The pulse sequence isrepeated 10 times, followed by a rest period for 55 minutes. Afterwards,the pulse sequence and rest period are repeated to a predeterminedvoltage. Furthermore, it has been found that FR6 type electrochemicalcells comprising wet milled FeS₂ particles maintain a voltage ≧1.2 forat least 180 minutes, desirably at least 210 minutes, and preferably atleast 230 minutes according to the 1500/650 mW 2/28 s×10 per hour DSCtest. FR6 type electrochemical cells utilizing wet milled FeS₂ particleshave a discharge time generally of at least 300 minutes, and preferablyat least 320 minutes to 1.05 volts according to the 1500/650 mW 2/28s×10 per hour DSC test. The measurements were performed at roomtemperature.

FR6 electrochemical cells prepared utilizing relatively small averageparticle size FeS₂ particles derived from the milling methods of thepresent invention such as wet or jet milling provide reduced anodevoltage values at varying depth of discharge percentages when comparedto prior art cells containing FeS₂ particles having an average sizegreater than or equal to about 22 micrometers as illustrated in FIG. 5.At 50% depth of discharge, the anode voltage for an electrochemical cellhaving FeS₂ particles of average particle size within the ranges of thepresent invention is less than 190 millivolts, desirably less than 170millivolts, preferably less than 100 millivolts, and most preferablyless than about 60 millivolts. At 25% depth of discharge the anodevoltage is less than 140 millivolts, desirably less than 120 millivolts,and preferably less than 75 millivolts. In order to obtain themeasurements, the cells were discharged using a Solartron 1470 availablefrom Solartron Analytical, Farnborough, England. The current was chosensuch that the current density was about 5 mA/cm². The cells were cycled2 minutes at 1 amp and 5 minutes at 0 amps. The cells were referenced byremoving the cell can bottom and suspending the cell in a beakercontaining electrolyte, in this case, 0.75 moles per liter solvent (9.1%by weight) lithium iodine in a solvent blend of 1,3-dioxolane,1,2-diethoxyethane and 3,5-dimethylisoxazole (63.1:27.6:0.20% byweight). The reference electrode, which is a strip of pure lithium metalin a syringe barrel with a Vycor tip, is located off to the side of thecell. The system is allowed to equilibrate for approximately 30 minutesbefore discharge. The measurements were performed at room temperature.

In a further embodiment, the FeS₂ utilized in the cathode of the cellsof the invention has at least a predetermined minimum pH before use in acell. Alternatively, a pH raising additive compound can be added to theFeS₂ to increase the pH of the FeS₂ to at least a predetermined minimumlevel. Natural FeS₂ is generally a mined commodity. Depending on thesource, lot, handling and processing equipment utilized to process theFeS₂, or the like, the FeS₂ can contain various naturally occurring orpossibly supplier added impurities in various amounts. Such impuritiescan include, but are not limited to, minerals or other naturallyoccurring compounds including elements such as Ag, Al, As, Ba, Ca, Cd,Co, Cr, Cu, Ga, Hg, K, Li, Mg, Mn, Na, Nb, Ni, Pb, Se, Si, Sn, Sr, Te,Ti, Tl, V, Zn and Zr in amounts which range from a few parts per billionto about a few parts per thousand of FeS₂ and quartz at less than about10 parts per 100 parts FeS₂. The amounts and types of impurities, aswell as other factors such as the amount of weathering or exposure ofFeS₂ to the atmosphere and FeS₂ ore processing conditions such as theuse of flotation are believed to contribute to the inherent pH of FeS₂.It is desirable to store the FeS₂ in a manner suitable to preventweathering and to minimize the storage time between FeS₂ processing anduse in cathode manufacture.

It has been unexpectedly discovered that incorporating (a) FeS₂ or (b)FeS₂ and a pH raising additive compound having at least a minimum pHvalue into a cathode of an electrochemical battery cell substantiallyprevents internal short circuits, or reduces the frequency and severityof internal short circuits. It is believed that use of the (a) FeS₂ or(b) FeS₂ and the pH raising additive compound of at least a minimum pHvalue prevents or substantially prevents internal short circuits bymaking the impurities, such as Lewis acids, present in the cell,especially in the FeS₂, less soluble and thus less able to formdendrites which can bridge the anode and cathode.

Alternatively, the pH raising additive compound can be included in otherinternal components of the cell, such as the electrolyte, but includingthe additive in the cathode is preferred.

The pH raising additive compounds are optionally added to the FeS₂ in aneffective amount to adjust the pH value to at least a predeterminedminimum value, in some embodiments. The pH raising additive compoundsare preferably bases or at least weak bases which can increase the pH ofthe FeS₂. In general, the stronger the base is, the less additiverequired. In one embodiment, pH raising additive compounds include oneor more Group IIA elements of the Periodic Table of the Elements, acidscavengers, or pH control agents, such as overbased metal, e.g. calcium,sulfonates; cycloaliphatic epoxides; organic amines; amino alcohols; orcombinations thereof. Classes of pH raising additive compounds includeoxides, hydroxides, and stearates.

Examples of suitable additive Group IIA element containing pH raisingadditive compounds include, but are not limited to, calcium oxide (CaO),calcium stearate (Ca(C₁₈H₃₅O₂)₂), calcium hydroxide (Ca(OH)₂), magnesiumoxide (MgO), strontium oxide (SrO), and barium oxide (BaO). The pHraising additive compound and thus the cathode and electrochemical cellof the present invention are free of the combination of calciumhydroxide and lithium carbonate.

Overbased calcium sulfonates which can be used as pH raising additivecompounds in one embodiment have the following formula:

wherein R is an aliphatic chain of 12-20 carbon units and n is greaterthan or equal to 20. The calcium carbonate portion of the structure isbelieved to be responsible for modifying the pH of the FeS₂. Overbasedcalcium sulfonates are available as EFKA® 6950 from Efka Additives,Heerenveen, Netherlands (Ciba Specialty Chemicals), as well as K-STAY®501 from King Industries, Norwalk, Conn.

Suitable organic amines which can be utilized as pH raising additivecompounds include, but are not limited to, diethylamine andtriethylamine.

Examples of suitable cycloaliphatic epoxies which can be utilized as pHraising additive compounds include, but are not limited to, butyleneoxide and soybean oil epoxide.

Amino alcohols suitable as pH raising additive compounds include, butare not limited to, 2-amino-2-methyl-1-propanol and2-dimethylamino-2-methyl-1-propanol.

The effective amount of pH raising additive compound utilized toincrease the pH of FeS₂ to at least a predetermined minimum value canvary depending on a number of factors including, but not limited to, theinherent pH value of FeS₂, target pH values for the FeS₂ and pH raisingadditive compound mixture, and effectiveness of the particular pHraising additive compound or compounds in adjusting pH of the FeS₂.

For example, if it was determined that FeS₂ had a pH of 3.5, theoreticalcalculations indicate 90 parts per million (ppm) of calcium oxide wouldbe needed to raise the pH to 7.0. Iron disulfide having a pH of 2.5would theoretically require 900 ppm calcium oxide to raise the pH to7.0. The above values were calculated given the following: pH=−log[H⁺],wherein [H⁺] is the proton concentration in a water suspension (not theFeS₂ itself). For the pH measurement as described above, 5 g of FeS₂ aresuspended in 50 mL of water as indicated to measure inherent pH of theFeS₂. This gives a “measurement dilution” ratio (mL solution/g pyrite)of 10. In this example, calcium oxide was utilized to raise pH.According to the formula CaO+2H⁺→Ca²⁺+H₂O, two moles of acid react withevery mole of calcium oxide. This gives a stoichiometry ratio of 0.5.The amount of pH raising additive compound, in this example calciumoxide, can be calculated using the formula: ppm pH raising additivecompound=[H⁺](measurement dilution ratio)(stoichiometry ratio)(FW)1000.The FW (formula weight) of CaO is 56.08. Accordingly, the amount ofcalcium oxide theoretically needed is calculated from the formula: ppmCaO=2.8×10⁵[H⁺].

In order to hinder or prevent dendrite formation and thus internal shortcircuits in electrochemical battery cells, the FeS₂ having an averageparticle size greater than or equal to 20 μm, utilized alone or incombination with a pH raising additive compound, has a pH value, priorto use in the cell, of generally from 4.0 to about 14.0, desirably from5.0 or 6.0 to 12.0, and preferably from 6.5 or 6.9 to 9.0. When the FeS₂has an average particle size of 19 μm or less, such as the describedabout 1 to about 19 μm, about 1.5 to about 10 or about 15 μm, or about 2to about 6 μm FeS₂ obtained utilizing a particle size reduction millingprocess described herein, the pH value of the FeS₂ or FeS₂ and pHraising additive compound is generally from 4.0 or 5.0 to about 14.0,desirably from 5.3 to 12.0, and preferably from 5.6 to 9.0. Relativelyhigh pH values achieved utilizing large amounts of pH raising additivecompound can reduce the active material content in the cathode and canaffect performance of the electrochemical battery cell. Accordingly, pHvalues should be closely monitored to balance battery characteristics.

Cathodes containing pH controlled FeS₂ can be prepared utilizing eithera wet or dry process, with the wet process being preferred. The term dryprocess refers to a process where the cathode active material mixturecomponents are mixed with the FeS₂ in dry form. The dry mix issubsequently formed into a cathode employing molding or other techniquesknow to those of ordinary skill in the art of battery manufacture. Thewet process, as described hereinabove, refers to a process wherein theactive material mixture components include at least one wetting agent,for example, a liquid such as an organic solvent.

In either process, a source of FeS₂ is obtained and the pH is determinedsuch as described hereinabove. If the inherent pH value is at least at apredetermined minimum level for the appropriate FeS₂ average particlesize, a cathode is prepared utilizing the FeS₂. As stated hereinabove,FeS₂ is obtained from suppliers having an average particle size greaterthan 20 μm. If the FeS₂ pH is less than the minimum value, one or morepH raising additive compounds are combined with the FeS₂ in an amountsufficient to provide a calculated pH of a mixture of the FeS₂ and pHraising additive compound of at least the required minimum value. Themixture of the FeS₂ and pH raising additive compound is subsequentlyutilized to form a cathode.

In one embodiment as described hereinabove, in addition to comprisingFeS₂ with an inherent pH, and optionally a mixture of FeS₂ and the pHraising additive compound with a calculated pH, of a predeterminedminimum value, the active material mixture can include additional activematerials, or other materials such as binders, conductive materials, orother additives. The desired active material mixture is subsequentlyapplied to the cathode substrate to form the cathode. If a wet cathodeforming process is utilized, a slurry of the active material mixture,preferably in a volatile organic solvent, is roll coated onto one orpreferably both sides of the cathode substrate. Thereafter, the coatingis dried to remove the solvent or other wetting agent. The coatedsubstrate is calendered to compact the coating, subsequently slit to thedesired width, and cut to the desired length.

In a preferred embodiment, it is desirable to utilize FeS₂ having anaverage particle size from 1 to 19 μm or range therewithin as describedabove, and further having at least a minimum predetermined pH in acathode for an electrochemical battery cell.

A preferred embodiment for preparing a cathode begins with obtainingFeS₂ with an average particle size greater than 20 μm. The inherent pHof the relatively large particle size FeS₂ is determined. A slurry isformed comprising a wetting agent, the FeS₂, and optionally a pH raisingadditive compound in an effective amount to provide a calculated pHvalue for the mixture of the FeS₂ and the pH raising additive compoundof at least a minimum predetermined level if the inherent pH of the FeS₂is less than the predetermined level. The slurry is then processed, suchas by milling described herein, in order to reduce the average FeS₂particle size to between 1 to 19 μm or a range therebetween. The milledslurry is applied to a cathode substrate to form a cathode. The cathodeis dried in a subsequent step and optionally further processed asdescribed.

As described herein, the cathode containing FeS₂ (and optionally a pHraising additive compound) having at least a predetermined minimum pHcan be combined with an anode comprising lithium, and also a separatorgenerally positioned between the anode and a cathode are placed in acell housing. A nonaqueous electrolyte is added to the cell and theanode, cathode, separator, and electrolyte are sealed in a cell housing.

The cathode is electrically connected to the positive terminal of thecell. This may be accomplished with an electrical lead, often in theform of a thin metal strip or a spring, as shown in FIG. 1. The lead isoften made from nickel plated stainless steel.

The separator is a thin microporous membrane that is ion-permeable andelectrically nonconductive. It is capable of holding at least someelectrolyte within the pores of the separator. The separator is disposedbetween adjacent surfaces of the anode and cathode to electricallyinsulate the electrodes from each other. Portions of the separator mayalso insulate other components in electrical contact with the cellterminals to prevent internal short circuits. Edges of the separatoroften extend beyond the edges of at least one electrode to insure thatthe anode and cathode do not make electrical contact even if they arenot perfectly aligned with each other. However, it is desirable tominimize the amount of separator extending beyond the electrodes.

To provide good high power discharge performance it is desirable thatthe separator have the characteristics (pores with a smallest dimensionof at least 0.005 μm and a largest dimension of no more than 5 μmacross, a porosity in the range of 30 to 70 percent, an area specificresistance of from 2 to 15 ohm-cm² and a tortuosity less than 2.5)disclosed in U.S. Pat. No. 5,290,414, issued Mar. 1, 1994, and herebyincorporated by reference. Suitable separator materials should also bestrong enough to withstand cell manufacturing processes as well aspressure that may be exerted on the separator during cell dischargewithout tears, splits, holes or other gaps developing that could resultin an internal short circuit.

To minimize the total separator volume in the cell, the separator shouldbe as thin as possible, but at least about 1 μm or more so a physicalbarrier is present between the cathode and anode to prevent internalshort circuits. That said, the separator thickness ranges from about 1to about 50 μm, desirably from about 5 to about 25 μm, and preferablyfrom about 10 to about 16 or about 20 μm. The required thickness willdepend in part on the strength of the separator material and themagnitude and location of forces that may be exerted on the separatorwhere it provides electrical insulation.

A number of characteristics besides thickness can affect separatorstrength. One of these is tensile stress. A high tensile stress isdesirable, preferably at least 800, more preferably at least 1000kilograms of force per square centimeter (kgf/cm²). Because of themanufacturing processes typically used to make microporous separators,tensile stress is typically greater in the machine direction (MD) thanin the transverse direction (TD). The minimum tensile stress requiredcan depend in part on the diameter of the cell. For example, for a FR6type cell the preferred tensile stress is at least 1500 kgf/cm² in themachine direction and at least 1200 kgf/cm² in the transverse direction,and for a FR03 type cell the preferred tensile strengths in the machineand transverse directions are 1300 and 1000 kgf/cm², respectively. Ifthe tensile stress is too low, manufacturing and internal cell forcescan cause tears or other holes. In general, the higher the tensilestress the better from the standpoint of strength. However, if thetensile stress is too high, other desirable properties of the separatormay be adversely affected.

Tensile stress can also be expressed in kgf/cm, which can be calculatedfrom tensile stress in kgf/cm² by multiplying the later by the separatorthickness in cm. Tensile stress in kgf/cm is also useful for identifyingdesirable properties related to separator strength. Therefore, it isdesirable that the separator have a tensile stress of at least 1.0kgf/cm, preferably at least 1.5 kgf/cm and more preferably at least 1.75kgf/cm in both the machine and transverse directions. For cells withdiameters greater than about 0.45 inch (11.4 mm), a tensile stress of atleast 2.0 kgf/cm is most preferable.

Another indicator of separator strength is its dielectric breakdownvoltage. Preferably the average dielectric breakdown voltage will be atleast 2000 volts, more preferably at least 2200 volts. For cylindricalcells with a diameter greater than about 0.45 in (11.4 mm), the averagedielectric breakdown voltage is most preferably at least 2400 volts. Ifthe dielectric breakdown voltage is too low, it is difficult to reliablyremove cells with defective or damaged separators by electrical testing(e.g., retention of a high voltage applied to the electrode assemblybefore the addition of electrolyte) during cell manufacturing. It isdesirable that the dielectric breakdown is as high as possible whilestill achieving other desirable separator properties.

The average effective pore size is another of the more importantindicators of separator strength. While large pores are desirable tomaximize ion transport through the separator, if the pores are too largethe separator will be susceptible to penetration and short circuitsbetween the electrodes. The preferred maximum effective pore size isfrom 0.08 μm to 0.40 μm, more preferably no greater than 0.20 μm.

The BET specific surface area is also related to pore size, as well asthe number of pores. In general, cell discharge performance tends to bebetter when the separator has a higher specific surface area, but theseparator strength tends to be lower. It is desirable for the BETspecific surface area to be no greater than 40 m²/g, but it is alsodesirable that it be at least 15 m²/g, more preferably at least 25 m²/g.

For good high rate and high power cell discharge performance a low areaspecific resistance is desirable. Thinner separators tend to have lowerresistances, but the separator should also be strong enough, limitinghow thin the separator can be. Preferably the area specific resistanceis no greater than 4.3 ohm-cm², more preferably no greater than 4.0ohm-cm², and most preferably no greater than 3.5 ohm-cm².

Separator membranes for use in lithium batteries are often made ofpolypropylene, polyethylene or ultrahigh molecular weight polyethylene,with polyethylene being preferred. The separator can be a single layerof biaxially oriented microporous membrane, or two or more layers can belaminated together to provide the desired tensile strengths inorthogonal directions. A single layer is preferred to minimize the cost.Suitable single layer biaxially oriented polyethylene microporousseparator is available from Tonen Chemical Corp., available from EXXONMobile Chemical Co., Macedonia, N.Y., USA. Setela F20DHI grade separatorhas a 20 μm nominal thickness, and Setela 16MMS grade has a 16 μmnominal thickness.

The anode, cathode and separator strips are combined together in anelectrode assembly. The electrode assembly may be a spirally wounddesign, such as that shown in FIG. 1, made by winding alternating stripsof cathode, separator, anode and separator around a mandrel, which isextracted from the electrode assembly when winding is complete. At leastone layer of separator and/or at least one layer of electricallyinsulating film (e.g., polypropylene) is generally wrapped around theoutside of the electrode assembly. This serves a number of purposes: ithelps hold the assembly together and may be used to adjust the width ordiameter of the assembly to the desired dimension. The outermost end ofthe separator or other outer film layer may be held down with a piece ofadhesive tape or by heat sealing.

Rather than being spirally wound, the electrode assembly may be formedby folding the electrode and separator strips together. The strips maybe aligned along their lengths and then folded in an accordion fashion,or the anode and one electrode strip may be laid perpendicular to thecathode and another electrode strip and the electrodes alternatelyfolded one across the other (orthogonally oriented), in both casesforming a stack of alternating anode and cathode layers.

The electrode assembly is inserted into the housing container. In thecase of a spirally wound electrode assembly, whether in a cylindrical orprismatic container, the major surfaces of the electrodes areperpendicular to the side wall(s) of the container (in other words, thecentral core of the electrode assembly is parallel to a longitudinalaxis of the cell). Folded electrode assemblies are typically used inprismatic cells. In the case of an accordion-folded electrode assembly,the assembly is oriented so that the flat electrode surfaces at oppositeends of the stack of electrode layers are adjacent to opposite sides ofthe container. In these configurations the majority of the total area ofthe major surfaces of the anode is adjacent the majority of the totalarea of the major surfaces of the cathode through the separator, and theoutermost portions of the electrode major surfaces are adjacent to theside wall of the container. In this way, expansion of the electrodeassembly due to an increase in the combined thicknesses of the anode andcathode is constrained by the container side wall(s).

A nonaqueous electrolyte, containing water only in very small quantitiesas a contaminant (e.g., no more than about 500 parts per million byweight, depending on the electrolyte salt being used), is used in thebattery cell of the invention. Any nonaqueous electrolyte suitable foruse with lithium and active cathode material the may be used. Theelectrolyte contains one or more electrolyte salts dissolved in anorganic solvent. For a Li/FeS₂ cell examples of suitable salts includelithium bromide, lithium perchlorate, lithium hexafluorophosphate,potassium hexafluorophosphate, lithium hexafluoro-arsenate, lithiumtrifluoromethanesulfonate and lithium iodide; and suitable organicsolvents include one or more of the following: dimethyl carbonate,diethyl carbonate, methylethyl carbonate, ethylene carbonate, propylenecarbonate, 1,2-butylene carbonate, 2,3-butylene carbonate, methylformate, γ-butyrolactone, sulfolane, acetonitrile,3,5-dimethylisoxazole, n,n-dimethyl formamide and ethers. Thesalt/solvent combination will provide sufficient electrolytic andelectrical conductivity to meet the cell discharge requirements over thedesired temperature range. Ethers are often desirable because of theirgenerally low viscosity, good wetting capability, good low temperaturedischarge performance and good high rate discharge performance. This isparticularly true in Li/FeS₂ cells because the ethers are more stablethan with MnO₂ cathodes, so higher ether levels can be used. Suitableethers include, but are not limited to acyclic ethers such as1,2-dimethoxyethane, 1,2-diethoxyethane, di(methoxyethyl)ether,triglyme, tetraglyme and diethyl ether; and cyclic ethers such as1,3-dioxolane, tetrahydrofuran, 2-methyl tetrahydrofuran and3-methyl-2-oxazolidinone.

Accordingly, various combinations of electrolyte salts and organicsolvents can be utilized to form the electrolyte for electrochemicalcells. The molar concentration of the electrolyte salt can be varied tomodify the conductive properties of the electrolyte. Examples ofsuitable nonaqueous electrolytes containing one or more electrolytesalts dissolved in an organic solvent include, but are not limited to, a1 mole per liter solvent concentration of lithiumtrifluoromethanesulfonate (14.60% by weight) in a solvent blend of1,3-dioxolane, 1,2-diethoxyethane, and 3,5-dimethyl isoxazole(24.80:60.40:0.20% by weight) which has a conductivity of 2.5 mS/cm; a1.5 moles per liter solvent concentration of lithiumtrifluoromethanesulfonate (20.40% by weight) in a solvent blend of1,3-dioxolane, 1,2-diethoxyethane, and 3,5-dimethylisoxazole(23.10:56.30:0.20% by weight) which has a conductivity of 3.46 mS/cm;and a 0.75 mole per liter solvent concentration of lithium iodide (9.10%by weight) in a solvent blend of 1,3-dioxolane, 1,2-diethoxyethane, and3,5-dimethylisoxazole (63.10:27.60:0.20% by weight) which has aconductivity of 7.02 mS/cm. Electrolytes utilized in the electrochemicalcells of the present invention have conductivity generally greater thanabout 2.0 mS/cm, desirably greater than about 2.5 or about 3.0 mS/cm,and preferably greater than about 4, about 6, or about 7 mS/cm.

Specific anode, cathode and electrolyte compositions and amounts can beadjusted to provide the desired cell manufacturing, performance andstorage characteristics.

The cell can be closed and sealed using any suitable process. Suchprocesses may include, but are not limited to, crimping, redrawing,colleting and combinations thereof. For example, for the cell in FIG. 1,a bead is formed in the can after the electrodes and insulator cone areinserted, and the gasket and cover assembly (including the cell cover,contact spring and vent bushing) are placed in the open end of the can.The cell is supported at the bead while the gasket and cover assemblyare pushed downward against the bead. The diameter of the top of the canabove the bead is reduced with a segmented collet to hold the gasket andcover assembly in place in the cell. After electrolyte is dispensed intothe cell through the apertures in the vent bushing and cover, a ventball is inserted into the bushing to seal the aperture in the cellcover. A PTC device and a terminal cover are placed onto the cell overthe cell cover, and the top edge of the can is bent inward with acrimping die to hold retain the gasket, cover assembly, PTC device andterminal cover and complete the sealing of the open end of the can bythe gasket.

The above description is particularly relevant to cylindrical Li/FeS₂cells, such as FR6 and FR03 types, as defined in International StandardsIEC 60086-1 and IEC 60086-2, published by the InternationalElectrotechnical Commission, Geneva, Switzerland. However, the inventionmay also be adapted to other cell sizes and shapes and to cells withother electrode assembly, housing, seal and pressure relief ventdesigns.

Features of the invention and advantages thereof are further illustratedin the following examples, wherein unless otherwise noted, experimentswere conducted at room temperature:

EXAMPLE 1

FR6 type cylindrical Li/FeS₂ cells with spirally wound electrodeassemblies were made with varying electrode assembly void volumes percentimeter of interfacial electrode assembly height over a range ofabout 0.373 to about 0.455 cm³/cm. The void volumes were varied byadjusting the volume of the voids within the active material mixturecoated on the cathode. This was done with various combinations ofmixture formulations, thickness and packing. The separator material usedin all cells was a highly crystalline, uniaxially oriented, microporouspolypropylene material with a 25 μm nominal thickness.

EXAMPLE 2

Samples of the cells from Example 1 were prepared for testing. For eachgroup with a given void volume per unit of height, some cells remainedundischarged and some cells were 50% discharged (discharged at a rate of200 mA for the time required to remove 50 percent of the ratedcapacity). Undischarged and 50% discharged cells were tested on anImpact Test, and the external temperature of each of the cells testedwas monitored during and for six hours after testing.

For the Impact Test a sample cell is placed on a flat surface, a 15.8 mmdiameter bar is placed across the center of the sample, and a 9.1 kgmass is dropped from a height of 61±2.5 cm onto the sample. The samplecell is impacted with its longitudinal axis parallel to the flat surfaceand perpendicular to the longitudinal axis of the 15.8 mm diameter barlying across the center of the cell. Each sample is subjected to only asingle impact.

None of the undischarged cells had an external temperature that exceeded170° C. The percentage of 50% discharged cells whose externaltemperatures exceeded 170° C. was plotted. The best curve fitting theplotted points is shown in FIG. 2, where the void volume per unit height(in cm³/cm) is on the x-axis, and the percentage of cells with anexternal temperature exceeding 170° C. is on the y-axis.

The Impact Test results show that as the electrode assembly void volumedecreases, the percentage of cells with an external temperatureexceeding 170° C. increases. From the graph in FIG. 2, 0% of the cellswith a void volume of approximately 0.45 cm³/cm of interfacial heightwould be predicted to have an external temperature exceeding 170° C.,and over 60% with a void volume of approximately 0.37 cm³/cm would bepredicted to exceed 170° C. The high external temperatures wereattributed to damage to the separator resulting in heat-generatinginternal short circuits.

Subsequent examination of both FR6 Li/FeS₂ cells after different levelsof discharge revealed that a net increase in the FR6 cell totalelectrode volume, which becomes greater as discharge proceeds, causesbending and buckling of the electrode strips and collapsing of thecentral core of the electrode assembly by the time the cells are 50%discharged. In contrast, similar examination of Li/MnO₂ cells withspirally wound electrodes showed little if any discernable change in theelectrode assembly at 50% discharge. The difference between the activematerial volumes and the volumes of the discharge reaction productsprovides an explanation for the difference in the effects of dischargeon the spirally wound electrode assemblies of Li/FeS₂ vs. Li/MnO₂ cells.

EXAMPLE 3

Four lots of FR6 cells were made, each with a separator made from adifferent material. A description of the separator materials is providedin Table 1, and typical separator properties, as determined by themethods described below, are summarized in Table 2. The separatormaterial used for Lot A is the same as that used in the cells inExample 1. Each cell contained about 1.60 g of electrolyte, theelectrolyte consisting of 9.14 weight percent LiI salt in a solventblend of 1,3-dioxolane, 1,2-dimethoxyethane and 3,5-dimethylisoxazole(63.05:27.63:0.18 by weight).

TABLE 1 Lot A Lot B Lot C Lot D highly highly crystalline amorphousamorphous crystalline uniaxially oriented biaxially oriented biaxiallyuniaxially microporous microporous oriented oriented polypropyleneultrahigh molecular microporous microporous 20 μm thick weightpolyethylene polyethylene polypropylene 20 μm thick 20 μm thick 25 μmthick

TABLE 2 Property (units) Lot A Lot B Lot C Lot D Porosity (%) 38 38 4240 Max. effective 0.10 0.06 0.38 0.10 pore size (μm) Dielectric 27002200 1600 2625 breakdown volt. (V) Tensile stress, 190 162 844 1336 TD(kgf/cm²) Tensile stress, 0.475 0.324 1.688 2.672 TD (kgf/cm) Tensilestress, 1687 2671 1541 1828 MD (kgf/cm²) Tensile stress, 4.218 5.3423.082 3.656 MD (kgf/cm) Tensile 1000 790 440 320 elongation, TD (%)Tensile 120 54 260 225 elongation, MD (%) Area specific 4.59 2.71 3.062.90 resist. (Ω · cm²) BET spec. surf. 44.0 48.9 16.2 36.4 area (m²/g)

The same cell design was used for all of Lots A-D. The cell design wasone with greater amounts of active materials, a higher concentration ofFeS₂ in the cathode mixture and an increased electrode interfacialsurface area, as well as a lower anode: cathode total input capacityratio, than cells from Example 1 with an electrode assembly void volumeto interfacial height ratio of about 0.452, resulting in a 22% increasein the cell interfacial capacity.

EXAMPLE 4

Cells from each lot in Example 3 were discharged 50% and then tested onthe Impact Test. The percentage of cells exceeding 170° C. on the testwas 20% for Lot A, 80% for Lot B and 0% for Lots C and D.

By increasing the interfacial capacity 22% compared to cells fromExample 1 with an electrode assembly void volume to interfacial heightratio of about 0.452, the percentage of cells exceeding 170° C. on theImpact Test increased from 0% to 20%. Cells from Lot A had a reducedamount of void space to accommodate a net increase in volume ofdischarge reaction products compared the volume of the unreacted activematerials, increasing the adverse effects of discharge on the Li/FeS₂electrode assembly observed in Example 2.

The reduced separator material thickness in Lot B compared to Lot Acontributed in a further increase in the percentage of cells exceeding170° C. on the Impact Test from 20% to 80%.

Although the thicknesses of the separator materials in Lots C and D werethe same as the thickness of the Lot B separator, there were no cells ineither Lot C or Lot D. The results for Lots C and D were comparable tothose for cells from Example 1 with an electrode assembly void volume tointerfacial height ratio of about 0.452, even though the void volumewithin the cathode and the separator material thickness were bothreduced in Lots C and D.

EXAMPLE 5

Three lots of FR6 cells were used to compare actual performance of FR6cells on relatively low rate and high rate discharge tests. The firstlot was Lot D from Example 3. Features of Lot D are summarized in Table3. The values listed are nominal values and can vary within typicalmanufacturing tolerances.

Cells in Lots E and F were made according to the prior art. The cells inLot F were like those in Example 1 with an electrode assembly voidvolume to interfacial height ratio of about 0.452. The features of LotsE and F are shown in Table 3. In Lot E the same separator material asthat in Lot F was used, but in Lot E the cathode mixture composition wasmodified and the cell interfacial capacity was increased by 18% comparedto Lot F. The use of a thinner (20 μm thick) separator in Lot D alloweda 22% increase in cell interfacial capacity compared to Lot F.

TABLE 3 Feature Lot D Lot E Lot F Anode Li—Al Li—Al Li—Al Li foil0.01524 0.01524 0.01524 thickness (cm) Li foil width (cm) 3.899 3.8993.861 Li foil cut 31.50 30.48 30.61 length (cm) Li foil weight (g) 0.990.97 0.95 Li input capacity/ 3859 3735 3664 cell (mAh) Anode interfacial3600 3485 3470 capacity/cell (mAh) Cathode Al current collector 0.002540.00254 0.00254 thickness (cm) Current collector 0.3313 0.3199 0.3186volume (cm³) Dry coating FeS₂ 92.00 92.00 92.75 (wt %): acetylene 1.401.40 2.5 black graphite 4.00 4.0 2.25 MX15 MX15 KS6 binder 2.00 2.0 2.00SEBS SEBS PEPP other 0.3 0.3 0.05 PTFE PTFE PEO other 0.3 0.3 silicasilica Coating real density 4.115 4.115 4.116 (g/cm³) Coating thickness0.0080 0.0080 0.0072 (ea. side) (cm) Coating loading 21.26 21.26 16.98(mg/cm²) Coating packing (%) 64 64 57 Coating width (cm) 4.077 4.0774.039 Cathode (coating) 29.85 28.83 28.96 length (cm) Coating weight/5.17 5.00 3.97 cell (g) Cathode input 4250 4110 3290 capacity/cell (mAh)Cathode interfacial 4005 3877 3105 capacity/cell (mAh) Separator (2pieces/cell) Material 20 μm PE 25 μm PP 25 μm PP Length/piece (cm) 39.539 39 Width/piece (cm) 4.4 4.4 4.4 Total volume (cm³) 0.431 0.425 0.532Electrode Assembly Winding mandrel 0.4 0.4 0.4 diameter (cm) Overwrapvolume (cm³) 0.124 0.124 0.124 Interfacial 3.899 3.899 3.861 height (cm)Can Ni pltd. Ni pltd. Ni pltd. steel steel steel Thickness (cm) 0.02410.0241 0.0241 Outside diameter (cm) 1.392 1.392 1.379 Inside diameter(cm) 1.344 1.344 1.331 Cell Internal void 10 10 12 volume (%)Anode/cathode 0.95 0.95 1.18 input capacity Interfacial 3600 3485 3105capacity (mAh) Cathode cap./interfac. 724 701 578 vol. (mAh/cm³)

EXAMPLE 6

Cells from each of Lots D, E and F were discharged at 200 mAcontinuously to 1.0 volt and at 1000 mA continuously to 1.0 volt. Table4 compares the results.

TABLE 4 Test Lot D Lot E Lot F  200 mA 3040 mAh 2890 mAh 2417 mAh 1000mA 2816 mAh 2170 mAh 2170 mAh

The following separator material properties are determined according tothe corresponding methods. Unless otherwise specified, all disclosedproperties are as determined at room temperature (20-25° C.).

-   -   Tensile stress was determined using an Instron Model 1123        Universal Tester according to ASTM D882-02. Samples were cut to        0.50 inches (1.27 cm) by 1.75 inches (4.45 cm). The initial jaw        separation was 1 inch (2.54 cm) and the strain rate was 2 inches        (5.08 cm) per minute. Tensile stress was calculated as applied        force divided by the initial cross sectional area (the width of        the sample perpendicular to the applied force times the        thickness of the sample).    -   Maximum effective pore diameter was measured on images made at        30,000 times magnification using a Scanning Electron Microscope        and covering an area of 4 μm×3 μm. For each separator sample, an        image was made of both major surfaces. On each image, the        largest pores were measured to determine the largest round        diameter that would fit within the pore wall (the maximum        effective diameter of the individual pores). The maximum        effective pore diameter of the sample was calculated by        averaging the maximum effective pore diameters of the two        largest pores on each side (i.e., the average of four individual        pores).    -   Porosity was determined by (1) cutting a sample of the        separator, (2) weighing the sample, (3) measuring the length,        width, and thickness of the sample, (3) calculating the density        from the weight and measurements, (4) dividing the calculated        density by the theoretical density of the separator polymer        resin, as provided by the separator manufacturer, (5)        multiplying the dividend by 100, and (5) subtracting this value        from 100.    -   Dielectric breakdown voltage was determined by placing a sample        of the separator between two stainless steel pins, each 2 cm in        diameter and having a flat circular tip, and applying an        increasing voltage across the pins using a Quadtech Model Sentry        20 hipot tester, and recording the displayed voltage (the        voltage at which current arcs through the sample).    -   Tensile elongation (elongation to break) was determined using an        Instron Model 1123 Universal Tester according to ASTM D882-02.        Samples were cut to 0.50 inches (1.27 cm) by 1.75 inches (4.45        cm). The initial jaw separation was 1 inch (2.54 cm) and the        strain rate was 2 inches (5.08 cm) per minute. Tensile        elongation was calculated by subtracting the initial sample        length from the sample length at break, dividing the remainder        by the initial sample length and multiplying the dividend by 100        percent.    -   Area Specific Resistance (ASR) was determined for separator        samples suspended in an electrolyte between two platinum        electrodes, using a Model 34 Conductance-Resistance Meter from        Yellow Springs Instrument, Yellow Springs, Ohio, USA, to make        resistance measurements. The electrolyte solution used was 9.14        weight percent LiI salt in a solvent blend of 1,3-dioxolane,        1,2-dimethoxyethane and 3,5-dimethylisoxazole (63.05:27.63:0.18        by weight). All testing was done in an atmosphere of less than 1        part per million of water and less than 100 parts per million of        oxygen. An electrically nonconductive sample holder, designed to        hold the separator sample with a 1.77 cm² area of the separator        exposed, was submerged in the electrolyte solution so that the        portion of the holder for holding the sample lay halfway between        two platinum electrodes, 0.259 cm apart. The resistance between        the electrodes was measured. The holder was removed from the        electrolyte, a separator sample inserted in the holder, and the        holder was slowly lowered into the electrolyte solution to the        same set level so that the sample was completely flooded with        electrolyte with no gas bubbles entrapped in the sample. The        resistance was measured. The ASR was calculated using the        formula:        ASR=A(R ₂ −R ₁ +ρL/A)        where A is the area of the exposed separator sample, R₂ is the        resistance value with the film present, R₁ is the resistance        value without the film, L is the separator sample thickness and        ρ is the conductivity of the electrolyte used.    -   Specific surface area was determined by the BET method, using a        TriStar gas adsorption analyzer from Micromeritics Instrument        Corporation, Norcross, Ga., USA. A sample of 0.1 g to 0.2 g of        the separator was cut into pieces of less than 1 cm² to fit the        sample holder, the sample was degassed under a stream of        nitrogen at 70° C. for 1 hour, and a pore size distribution        analysis was performed using nitrogen as the adsorbant gas and        collecting full adsorption/desorption isotherms.

EXAMPLE7

Cylindrical FR6 type lithium/FeS₂ cells having spirally wound electrodeassemblies were constructed with varying average particle size FeS₂particles of 22 μm (control), coarse size FeS₂ of 75 μm, media milledFeS₂ between 5 and 10 μm (calculated estimate), and jet milled FeS₂ of4.9 μm. The cells were identical to the cells of Lot D of Table 3 exceptfor FeS₂ average particle size and typical and expected processvariation. FIGS. 3 a and 3 b are SEM photographs of cross sections ofcoated cathodes made with conventional (unmilled) and media milledcathode slurry mixtures, respectively.

The discharge time of each cell was tested using the 1500/650 mW 2/28s×10 per hour DSC test described hereinabove. The results areillustrated in Tables 5a and 5b. Two sets of tests were conducted withthe media milled FeS₂-containing cells.

TABLE 5a Average Particle Size FeS₂ Coarse Control Jet Milled ServiceFEP MV = 75 μm MV = 22 μm MV = 4.9 μm Improvement 1.2 V  37 min. 194min. 296 min. 1.52 1.1 V 175 min. 288 min. 332 min. 1.15 1.05 V  214min. 314 min. 340 min. 1.08 1.0 V 243 min. 331 min. 345 min. 1.04

TABLE 5b Service Control Media Milled Control Media Milled FEP 22 μm5-10 μm Improvement 22 μm 5-10 μm Improvement 1.2 V 188 min. 236 min.1.25 184 min. 230 min. 1.25 1.1 V 281 min. 311 min. 1.11 277 min. 304min. 1.10 1.05 V  305 min. 329 min. 1.08 300 min. 322 min. 1.07 1.0 V318 min. 338 min. 1.06 314 min. 331 min. 1.05

As evident from Tables 5a and 5b, it is illustrated that cells preparedwith the media and jet milled FeS₂ particles provide a substantiallylonger discharge time to 1.05 volts when compared to the prior artcontrol FeS₂ particles of 22 μm average particle size and coarse sizedFeS₂ particles of 75 μm average particle size. The media milledFeS₂-containing cells also maintained a cut voltage of ≧1.2 for anaverage 69.6% of service time ≧1 volt, whereas the control onlymaintained such voltage for an average of 58.9% of service time.Likewise, the jet milled FeS₂-containing cell maintained a cut voltageof ≧1.2 for 85.7% of discharge time ≧1 volt.

EXAMPLE 8

FR6 type cylindrical lithium/FeS₂ cells with spirally wound electrodeassemblies were constructed. Average FeS₂ particle size, electrolytecomposition, and separator thickness were varied as set forth in Table6. The rest of the cell features were the same as described for Lot D ofTable 3 except for typical and expected process variation. Cells 1-4represent prior art cells.

TABLE 6 Cell 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 FeS₂ average 22 μm5-10 μm* 22 μm 5-10 μm particle size (media milled) (media milled)Separator 50 25 20 16 50 25 20 16 50 25 20 16 50 25 20 16 thickness (μm)Electrolyte 1.5 moles per liter solvent lithium 0.75 moles per litersolvent lithium iodide (9.1% trifluoromethanesulfonate (20.4% by wt.) inby wt.) in 1,3-dioxolane, 1,2-diethoxy ethane, 1,3-dioxolane,1,2-diethoxyethane, and and 3,5 dimethylisoxazole (63.1:27.6:0.2% bywt.) 3,5-dimethylisoxazole (23.1:56.4:0.2% by wt.) *(calculatedestimate)

Each cell was tested utilizing the 1500/650 mW 2/28 s×10 per hour DSCtest. The effect of electrolyte-separator resistance and FeS₂ particlesize is illustrated in FIG. 4. Plots of the cell groups illustrate thatreductions in separator thickness, use of relatively small averageparticles size FeS₂ particles, as well as the type of electrolyteindividually effect cathode efficiency. In FIG. 4, the lower most linerepresents a plot of a best fit line for experimental results for cells1-4. Likewise, the remaining lines in ascending order represent resultsfor cells 5-8, 9-12, and 13-16, respectively.

EXAMPLE 9

The anode voltage of FR6 type cylindrical lithium/FeS₂ cells havingspirally wound electrode assemblies was measured over the life of thecells. The cells were of substantially identical construction as setforth in Lot D of Table 3 except that one cell was constructed of FeS₂average particle of 22 μm, a second cell utilized media milled FeS₂particles of average size between 5 and 10 μm (calculated estimate), andthe third cell utilized jet milled FeS₂ particles of 4.9 μm average sizeand typical and expected process variation. The anode voltage of eachcell was plotted as a function of depth of discharge as set forth inFIG. 5. Full cell voltage as a function of depth of discharge is plottedin FIG. 6. Testing procedures have been set forth hereinabove.

At 50% depth of discharge, the anode voltage is reduced by 40 millivoltswhere the average particle size is reduced from 22 to 5.2 μm. Reducingthe average FeS₂ particle size from 22 μm to 4.9 μm reduced the anodevoltage 150 millivolts. L92 size electrochemical cells were constructedand tested in a similar manner. It was discovered that utilizing FeS₂ ofaverage particle size ranges disclosed herein increase the overall cellvoltage at any given depth of discharge independent of cell size.

EXAMPLE 10

The FeS₂ average particle size, while strongly influencing high cellperformance at standard ambient conditions, has an even greaterinfluence at low temperature. The following Table 7 compares twodifferent studies of media milled cathodes of average particle sizebetween 5 and 10 μm (calculated estimate) and control FeS₂ of averageparticle size 22 μm, and cell performance as a function of temperature.The cells were constructed substantially the same as described for Lot Dof Table 3 except for typical and expected process variation. The testis a proposed standard simulated DSC—ANSI application previously defined(1500 mW/650 mW) to 1.05 volts. While reducing particle size improvesperformance by 5% or more at ambient conditions, improvements of over600% are observed at −20° C.

TABLE 7 Con- Media Con- Media Temper- trol Milled Performance trolMilled Performance ature Min. Min. Ratio Min. Min. Ratio 21° C. 304 3251.07 302 318 1.05 0° C. 178 227 1.27 186 121 1.14 −20° C. 14 102 7.28 16100 6.25

EXAMPLE 11

Two lots of Li/FeS₂ cells were assembled, one (Lot G) substantiallyidentical to Lot D (Tables 2 and 3), and the other (Lot H) was like LotG except for the cathode mixture composition. The FeS₂ used for bothlots had an inherent pH of 3.9 as received from the supplier and wassubsequently media milled with a bench top media mill using 1.2 to 1.7mm diameter Zirconox media to produce a bimodal particle sizedistribution, one mode with an average diameter of about 2 μm and theother mode, of similar distribution, with an average diameter of about10 μm, as determined by acoustic attenuation, using a Model DT1200spectrometer from Dispersion Technology, Inc., Bedford Hills, N.Y., USA.The cathode mixture used in Lot H contained 91.3 weight percent FeS₂,1.40 weight percent acetylene black, 4.00 weight percent graphite, 2.00weight percent binder, 0.3 weight percent PTFE, 0.3 weight percentsilica and 0.7 weight percent CaO. It differed from the cathode mixtureused for Lot G in that CaO was substituted for a portion of the FeS₂,the amount of CaO being an amount calculated to be sufficient to raisethe pH of the FeS₂ from 3.9 to about 12.4. After the electrodesassemblies were wound and inserted into cans, the addition ofelectrolyte to the cells was delayed for three days to exaggeratedetrimental effects of FeS₂ weathering.

After the cells were filled with electrolyte, sealed and predischarged,cells from both lots were stored for one week at 71° C. and then testedfor OCV. The OCV distributions are shown in Table 8. Thirty percent ofthe cells in Lot G had OCV's below 1.80 volts, while all cells in Lot Hwere above 1.80 volts.

TABLE 8 OCV Interval Midpoint Lot G Lot H 1.88 — — 1.86 5 17 1.84 4 11.82 2 2 1.80 2 — 1.78 3 — 1.76 — — 1.74 1 — 1.72 1 — 1.70 2 —

Accordingly, cells of the present invention including FeS₂ having atleast a predetermined minimum pH value were shown to possess desirableopen current voltage values, and thus no internal short circuits, whencompared to a control formulation having an inherent pH below thepredetermined minimum pH value.

It will be understood by those who practice the invention and thoseskilled in the art that various modifications and improvements may bemade to the invention without departing from the spirit of the disclosedconcepts. The scope of protection afforded is to be determined by theclaims and by the breadth of interpretation allowed by law.

What is claimed is:
 1. An electrochemical battery cell, comprising: ahousing; a negative electrode comprising lithium; a positive electrodecoated onto a current collector, the positive electrode comprising amixture of iron disulfide and a pH raising additive compound, whereinthe pH raising additive compound is present in an effective amount toprovide the mixture with a calculated pH of 6.5 to 14.0 and wherein theiron disulfide has a purity level of 95%; wherein the pH raisingadditive compound is selected from the group consisting of: an organicamine, a cycloaliphatic epoxide, an amino alcohol, an overbased metalsulfonate, triethylamine, diethylamine, butylene oxide, soybean oilepoxide, 2-amino-2-methyl-1-propanol and combinations thereof; and anon-aqueous electrolyte mixture comprising at least one salt dissolvedin a solvent disposed within the housing.
 2. The electrochemical batterycell according to claim 1, wherein the calculated pH of the irondisulfide and pH raising additive compound mixture is 6.9 to 14.0. 3.The electrochemical battery cell according to claim 1, wherein the irondisulfide has an average particle size of from 1 to 19 μm.
 4. Theelectrochemical battery cell according to claim 3, wherein the irondisulfide has an average particle size of from 1.5 to 15 μm.
 5. Theelectrochemical battery cell according to claim 2, wherein the irondisulfide has an average particle size of from 20 to 30 μm.
 6. Theelectrochemical battery cell according to claim 1, wherein the irondisulfide comprises greater than 80% of a positive electrode activematerial, and wherein the negative electrode, the positive electrode,and the separator form a spiral wound cylindrical electrode assembly,with a radial outer surface disposed adjacent an inner surface of a sidewall of the housing.
 7. The electrochemical battery cell according toclaim 3, wherein the iron disulfide comprises greater than 95% of apositive electrode active material, and wherein an electrode gap betweenadjacent negative and positive electrodes is from 15 μm to 45 μm.
 8. Theelectrochemical battery cell according to claim 7, wherein the negativeelectrode, the positive electriode, and the separator form a spiralwound cylindrical electrode assembly, with a radial outer surfacedisposed adjacent an inner surface of a side wall of the housing,wherein the positive electrode comprises a current collecting substratewith a coating on each side of the substrate, said coating comprisingthe active material wherein each coating has a thickness of 10 to 100μm, and wherein the separator has a thickness from 5 to 25 μm.
 9. Anelectrochemical battery cell, comprising: a housing; a negativeelectrode comprising lithium; a positive electrode coated onto a currentcollector the positive electrode comprising a mixture of iron disulfideand a pH raising additive compound, wherein the pH raising additivecompound is present in an effective amount to provide the mixture with acalculated pH of 5.6 to 14.0, wherein the iron disulfide has an averageparticle size from 1 to 19 μm; wherein the pH raising additive compoundis selected from the group consisting of: an organic amine, acycloaliphatic epoxide, an amino alcohol, an overbased metal sulfonate,triethylamine, diethylamine, butylene oxide, soybean oil epoxide,2-amino-2-methyl-1-propanol and combinations thereof; and a non-aqueouselectrolyte mixture comprising at least one salt dissolved in a solventdisposed within the housing.
 10. The electrochemical battery cellaccording to claim 9, wherein the iron disulfide comprises greater than80% of a positive electrode active material, and wherein the negativeelectrode, the positive electrode, and the separator form a spiral woundcylindrical electrode assembly, with a radial outer surface disposedadjacent an inner surface of a side wall of the housing.
 11. Theelectrochemical battery cell according to claim 10, wherein the irondisulfide comprises greater than 95% of the positive electrode activematerial, and wherein an electrode gap between adjacent negative andpositive electrodes is from 15 to 45 μm.
 12. The electrochemical batterycell according to claim 11, wherein the negative electrode, the positiveelectrode, and the separator form a spiral wound cylindrical electrodeassembly, with a radial outer surface disposed adjacent an inner surfaceof a side wall of the housing, wherein the positive electrode comprisesa current collecting substrate on a coating on each side of thesubstrate, said coating comprising the active material wherein eachcoating has a thickness 10 to 100 μm, and wherein the separator has athickness from 1 to 50 μm.
 13. A process for making a cathode,comprising the steps of: obtaining iron disulfide; determining a pH ofthe iron disulfide; and (a) if the iron disulfide pH is from 6.9 to 14.0and an average iron disulfide particle size is 20 to 30 μm, coating acathode onto a current collector using the iron disulfide; (b) if theiron disulfide pH is from 5.6 to 14.0 and an average iron disulfideparticle size is 1 to 19 μm, forming a cathode using the iron disulfide;(c) if the iron disulfide pH is less than 6.9 and an average irondisulfide particle size is greater than 20 μm, forming a cathode bymixing the iron disulfide and an effective amount of a pH raisingadditive compound selected from the group consisting of: an organicamine, a cycloaliphatic epoxide, an amino alcohol, an overbased metalsulfonate, triethylamine, diethylamine, butylene oxide, soybean oilepoxide, 2-amino-2-methyl-1-propanol and combinations thereof to providea calculated pH of the iron disulfide and pH raising additive compoundof 6.0 to 14.0 and coating the mixture onto a current collector; and (d)if the iron disulfide pH is less than 5.6 and an average iron disulfideparticle size is 1 to 19 μm, forming a cathode by mixing the irondisulfide and an effective amount of a pH raising additive compoundselected from the group consisting of: an organic amine, acycloaliphatic epoxide, an amino alcohol, an overbased metal sulfonate,triethylamine, diethylamine, butylene oxide, soybean oil epoxide,2-amino-2-methyl-1-propanol and combinations thereof to provide acalculated pH of the iron disulfide and pH raising additive compound of5.0 to 14.0 and coating the mixture onto a current collector.
 14. Theprocess for making a cathode according to claim 13, further includingthe steps of creating and milling a slurry including the iron disulfideto reduce the average iron disulfide particle size to 1 to 19 μm,applying the milled slurry to the current collector to form the cathodecoating, and drying the cathode.
 15. A process for making anelectrochemical battery cell, comprising the steps of: combining thecathode of the process of claim 13 with an anode comprising lithium anda separator situated between the anode and the cathode; adding anon-aqueous electrolyte; and sealing the anode, cathode, separator andelectrolyte in a cell housing.
 16. A process for making a cathode,comprising the steps of: obtaining iron disulfide; mixing a pH raisingadditive compound selected from the group consisting of: an organicamine, a cycloaliphatic epoxide, an amino alcohol, an overbased metalsulfonate, triethylamine, diethylamine, butylene oxide, soybean oilepoxide, 2-amino-2-methyl-1-propanol and combinations thereof with theiron disulfide in an effective amount so that a calculated pH of theiron disulfide and pH raising additive compound is 6.9 to 14.0; andadding a binder and a conductive additive to the mixture of irondisulfide and the pH raising additive compound and coating the resultingmixture onto a cathode substrate.
 17. The process for making a cathodeaccording to claim 16, further including the steps of forming a slurrycomprising a wetting agent, the iron disulfide, and the pH raisingadditive compound, and applying the slurry to the cathode substrate, anddrying the substrate.
 18. A process for making an electrochemicalbattery cell, comprising the steps of: spirally winding the cathode ofthe process of claim 16 with an anode consisting essentially of lithiumor a lithium alloy and a separator situated between the anode and thecathode; adding a non-aqueous electrolyte; and sealing the anode,cathode, separator and electrolyte in a cell housing.